One FSS embodiment is FSW. FSW is a joining process where welding is accomplished via mechanical stirring at a temperature that is below the melting point of the material being welded. In FSW, the welding tool comprises a shoulder and a pin (also called a probe). The tool is rotated while traversing the weld line. The shoulder applies pressure and friction induced heat to the surface of the material while the probe plunges into the material and induces material flow.
Another FSS embodiment is referred to as friction stir processing (FSP). FSP is a surface treatment technique which generally uses the same tool body as FSW, but lacks a FSW type probe. FSP is a relatively new surface-engineering technology that can locally eliminate or reduce casting defects and refine microstructures, thus improving strength and ductility, increasing resistance to corrosion and fatigue, enhancing formability, and improving other properties. FSP can also produce fine-grained microstructures through the thickness, imparted by superplasticity. Analogous to FSW, in FSP there is generally substantial and complex material flow.
FSW can be applied in a number of configurations, or positions, of the material to be welded and the FSW tool. One FSW configuration is referred to as butt-weld. In this arrangement the materials to be jointed are generally butted together side by side. The tool traverses the seam between the two samples, welding them together. The probe generally extends nearly to the base of the material. In this case, it is desirable for the probe to be well positioned with respect to the weld line.
Another FSW configuration is referred to as lap weld. In this case, the two samples are laid one on top of the other. The FSW probe extends through the top material and some distance into the second to accomplish the weld. Another FSW configuration is referred to as T-joint. In the T-joint, the materials to be welded are arranged in a T, with a horizontally-oriented sample set on top of a vertically-oriented sample. Other FSW configurations include the corner weld and edge weld.
As known in the art, welds from FSW can experience a loss in weld quality when the system parameters and conditions are not well set. For instance, in the case of FSW, loss of quality can occur if the welding tool rotates too slowly, or too quickly, or if the probe is not the correct length for the material (e.g. a probe in butt-welding extends only halfway through the material, leaving the lower half of the seam un-bonded).
One flaw-inducing condition for FSW and FSP systems can be due to lateral misalignment during the FSW or FSP process. Lateral misalignment results when the FSW or FSP tool is offset relative to the selected lateral position or selected path referenced to the workpiece(s). In FSW, lateral misalignment can cause poor quality welds. In FSP, lateral misalignment can cause unintended microstructural results.
In the case of FSW, examples of lateral misalignment for lap weld, T-weld and butt weld (left to right) are shown in FIG. 1 as A, B and C, respectively. In lap-welding, lateral misalignment can arise when the probe is located entirely within the material but the shoulder of the tool is not completely in contact with the upper material. This condition can result in a number of generally undesirable consequences that result from insufficient heat input due to less shoulder contact and thus less friction. Also material could be ejected out into the area under the exposed shoulder. In the T-joint (FIG. 1B), the probe is shown offset to the point where a portion of the probe does not reside in the lower material. In T-joints, the geometry of the material beneath the shoulder is changing with lateral offset, in that the tool is more or less centered over the vertical member. Additionally, the probe moves toward or away from the edges of the vertical member. These changing conditions will cause changes in weld quality. Lateral offset is known to cause deterioration of quality for T-joints in both the extended un-bonded region to the right of the probe as well as the loss of the material which is generally ejected to the left of the probe. Finally in the butt-joint (FIG. 1C), the probe is shown laterally offset relative to the joint line sufficiently so that the probe is only in one of the material pieces. In butt-joints, the composition of the material under the shoulder does not change, but the amount the probe is in each material does. At centered locations, the probe is half in one material and half in another. In an offset position, it is largely in one piece, and only slightly in the other. The resulting weld quality for significant lateral offset in butt-joints will likely be quite low.
Visualization is one known technique for identifying and correcting lateral misalignment. In butt-joint and in some lap welding configurations the alignment of the FSW tool with regard to the weld seam can generally be visually observed. However, in other lap welds including blind T-joint, visualization is not generally possible. In blind T-joints this inability results because the lower vertical member cannot be seen through the upper horizontal member. Similarly, in certain FSP processes, visualization is not generally possible. There is thus a need for a new technique to better maintain FSW and FSP processing tools in a desired lateral alignment during system operation.